Scientists have been studying molecular electronics in an effort to circumvent the size limitations on electronic components. This has been done in part because molecular electronics has increased flexibility and ease of processing with extremely high density information processing. Although some niche applications have been found for such materials, many problems such as robustness, processability, stability, and addressability still exist. While smaller is very often better, such devices have limited bandwidth due to the capacitance of electronic circuits. Conversely, optical information processing holds promise for significantly higher bandwidth devices, but suffers from even more severe size and address ability concerns than those that limit conventional electronics. These problems result from the diffraction limit—the spatial extent of light in a medium of refractive index n is limited by diffraction to about λ/2n, where λ is the free space wavelength of light. Thus, although the construction of conventional waveguides from high index materials enables the minimum beam size to be decreased significantly, waveguides are typically several times this diameter to adequately confine light via total internal reflection (TIR). Both modern lithography methods and molecular electronics have demonstrated success at alleviating this size constraint for purely electronic devices, but diffraction imposes a fundamental size limit in further shrinking devices for optical information processing.
The ability to transport optical signals through structures that are smaller than the free-space optical wavelength relies upon one of two physical processes taking place. One technique is for a waveguide to be constructed of an extremely high refractive index material. This technique can be accomplished in a simple fashion by using high index glasses to form optical fibers, but even the highest indices (n is about 3) shrink the limiting dimensions to λ/6, or about 100 nm for visible light. Since the range of angles capable of propagating in such fibers is smaller than that in a tightly focused spot, realistic visible waveguide dimensions of about 400 nm should be attainable. Thus, while optical fibers are nearly ideal for low loss, long range optical communication, the size constraints imposed by diffraction limits their incorporation into future nanoscale optical devices.
Alternatively, 30 nm diameter metallic structures have been theoretically proposed to confine and transmit light due to the large negative dielectric constants. Because light in such materials has imaginary transverse wave vectors, the minimum waveguide diameter can be made arbitrarily small. However, as such structures shrink, the metallic structures exhibit exponentially increasing losses due to both the negative (absorptive) and imaginary (imperfectly conductive) portions of the dielectric constant. Since the optical dielectric constant, ∈, is the square of the complex refractive index, and large negative dielectric constants result from the large imaginary refractive index of the material, light propagating in such devices will be strongly attenuated because of absorption, rendering them impractical for device fabrication.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.